US10931114B2 - Controlling a voltage source converter in a DC system - Google Patents

Controlling a voltage source converter in a DC system Download PDF

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US10931114B2
US10931114B2 US16/635,025 US201716635025A US10931114B2 US 10931114 B2 US10931114 B2 US 10931114B2 US 201716635025 A US201716635025 A US 201716635025A US 10931114 B2 US10931114 B2 US 10931114B2
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converter
voltage source
source converter
control signal
voltage
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US20200251906A1 (en
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Ying Jiang-Häfner
Adil ABDALRAHMAN
Per Holmberg
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Hitachi Energy Ltd
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ABB Power Grids Switzerland AG
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
    • H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
    • H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/75—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means
    • H02M7/757—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only
    • H02M7/7575—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only for high voltage direct transmission link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H02J2003/365—Reducing harmonics or oscillations in HVDC
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
    • H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
    • H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/75—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means
    • H02M7/757—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Definitions

  • the present invention relates to a first voltage source converter and a converter station comprising the first voltage source converter as well as to a method and computer program product for controlling the first voltage source converter.
  • Voltage source converters are often used for converting between alternating current (AC) and Direct Current (DC), such as between three-phase AC and DC.
  • a converter is then typically provided in a converter station and has a DC side connected to a DC system and an AC side connected to an AC system.
  • the converter is then made up of a number of phase legs, one for each phase, where each phase leg comprises two phase arms, an upper and a lower phase arm.
  • Each phase arm then comprises a corresponding converter valve, where the valves of a phase leg are together used for forming an AC waveshape.
  • the DC system may be a bipole DC system, i.e. a system comprising two DC poles; a positive pole and a negative pole.
  • the converter station comprises two converters; one having its DC side connected to the positive pole and another having its DC side connected to the negative pole.
  • both the converters may have their AC sides interconnected for instance via a common AC bus linking to or being a part of the AC system.
  • the two converters of the converter station operate in a frequency and voltage control mode.
  • the two poles share the active power via a power frequency droop. They also share the AC voltage control via a voltage-reactive power droop.
  • the main advantage of operating both converters in the frequency and voltage control mode is that the two poles can automatically share the active power imbalance (between load and generation) during grid restoration, or active power generated by wind farms, thanks to the power frequency droop functionality.
  • the converter station is a black starting station in which both converters operate in frequency and voltage control mode and being connected to a further supporting station via the DC system it is possible that the two converters start to oscillate against each other.
  • the invention is concerned with the problem of damping such oscillations between two converters connected to the same DC system and interconnected on the AC side.
  • the present invention is directed towards damping oscillations between a first and a second voltage source converter when the converters have interconnected AC sides and DC sides connected to a DC system.
  • a first voltage source converter having a direct current, DC, side for connection to a DC system and having an alternating current, AC, side for connection to an AC system and for interconnection with an AC side of a second voltage source converter, the second voltage source converter having a DC side connected to the DC system and the first voltage source converter comprising:
  • control unit operative to control the converter valves to generate at least one AC waveform and to reduce oscillations between the converters.
  • This object is according to a second aspect achieved through a converter station comprising a first voltage source converter having a direct current, DC, side for connection to a DC system and having an alternating current, AC, side for connection to an AC system and for interconnection with an AC side of a second voltage source converter, the second voltage source converter having a DC side for connection to the DC system, where the first voltage source converter is the first voltage source converter according to the first aspect.
  • the object is according to a third aspect achieved through a method of controlling a first voltage source converter having a direct current, DC, side connected to a DC system, having an alternating current, AC, side connected to an AC system and being interconnected with an AC side of a second voltage source converter, the second voltage source converter having a DC side connected to the DC system and the first voltage source converter comprising a number of converter valve pairs, each pair being connected to a corresponding AC phase of the AC system, the method being performed in a control unit of the first voltage source converter and comprising
  • controlling the converter valves to generate at least one AC waveform and to reduce oscillations between the converters.
  • the object is according to a fourth aspect of the present invention achieved through a computer program product for controlling a first voltage source converter having a direct current, DC, side connected to a DC system, having an alternating current, AC, side connected to an AC system and being interconnected with an AC side of a second voltage source converter, the second voltage source converter having a DC side connected to the DC system and the first voltage source converter comprising a number of converter valve pairs, each pair being connected to a corresponding AC phase of the AC system, the computer program product comprising a data carrier with computer program code configured to cause a control unit of the first voltage source converter to
  • the first and second converters may provided in the same converter station in a bipole DC system or converters in different converter stations in a monopole DC system.
  • the present invention has a number of advantages. It enables efficient oscillations damping of power swings between the two converters. Moreover, as the damping is performed using control it is possible to avoid introducing potentially expensive components having losses, such as resistors. Resistors are also bulky and thus increase the footprint of the converter. The degree of damping may also be flexibly set. The control is also easily introduced in a converter through merely adding some software.
  • FIG. 1 schematically shows two converter stations connected to separate AC systems and interconnected by a DC system
  • FIG. 2 schematically shows a first converter of a first of the converter stations
  • FIG. 3 schematically shows a control unit for controlling the first converter comprising a waveform control module and an active damping control module,
  • FIG. 4 schematically shows a first variation of a d component control section of the active damping control module
  • FIG. 5 schematically shows a corresponding q component control section of the active damping control module
  • FIG. 6 schematically shows a second variation of the d component control section
  • FIG. 7 schematically shows a computer program product in the form of a data carrier comprising computer program code for implementing the control unit.
  • FIG. 1 shows a single line diagram of two converter stations 10 and 12 , each connected to a separate Alternating Current (AC) system and being interconnected by a Direct Current (DC) system, which may be a High Voltage Direct Current (HVDC) power transmission system.
  • the DC system is a bipole system and therefore there is a first pole 22 and a second pole 24 .
  • the first converter station 10 comprises a first and second converter 14 and 16 and the second converter station 12 comprises a third and a fourth converter 18 and 20 .
  • the converters convert between AC and DC and therefore each converter has a DC side facing the DC system and an AC side facing an AC system.
  • the first converter 14 of the first converter station 10 is connected to a first AC bus ACB 1 via a first transformer T 1 and the second converter 16 is also connected to the first AC bus ACB 1 via a second transformer T 2 , where the first AC bus ACB 1 may be a part of or considered to be a first AC system. It can be seen that because of this, the AC sides of the first and the second converters 14 and 16 are interconnected via the first and second transformers T 1 and T 2 and the first AC bus ACB 1 .
  • the third converter 18 of the second converter station 12 is connected to a second AC bus ACB 2 via a third transformer T 3 and the fourth converter 20 is connected to the second AC bus ACB 2 via a fourth transformer T 4 , where the second AC bus ACB 2 may be a part of or considered to be a second AC system.
  • the first converter 14 is also on the DC side connected to the third converter 18 via the first pole 22 and the second converter 16 is connected to the fourth converter 20 via the second pole 24 , where the first and second poles 22 and 24 may be realized as cables, such as subsea or submarine cables, or overhead lines.
  • the first converter 14 is also interconnected with the second converter 16 on the DC side, where this interconnection is grounded, typically via an electrode line and a grounding electrode.
  • the third converter 18 is interconnected with the fourth converter 20 on the DC side, where also this interconnection is grounded, for instance via an electrode line and a grounding electrode.
  • the second AC system may be a system where energy is generated, and the first AC system may be a system where energy is consumed.
  • the AC power systems may also be three-phase systems.
  • the poles 22 and 24 may also cover long distances for transferring power.
  • the DC system is here a point-to-point bipole DC system interconnecting two converter station. It should be realized that as an alternative it could be a multi-terminal DC system, like a multi-terminal HVDC system, comprising more converter stations. It is also possible that the DC system is a monopole HVDC system, in which case the first and second converter may be provided in different converter stations but still be interconnected on the AC side.
  • FIG. 2 shows one way of realizing the first converter 14 .
  • the exemplifying first converter 14 is a three-phase voltage source converter for converting between AC and DC and has, as was described earlier, a DC side facing the DC system and an AC side facing the first AC system.
  • the first converter 14 therefore comprises three phase legs PL 1 , PL 2 and PL 3 , for instance connected in parallel between a first and a second DC terminal DC 1 and DC 2 , where the first DC terminal DC 1 may be connected to the first pole 22 and the second DC terminal may be connected to a first DC terminal of the second converter 16 , in which case a second DC terminal of the second converter 16 would be connected to the second pole 24 .
  • the interconnection between the second DC terminal DC 2 of the first converter 14 and the first DC terminal of the second converter may in this case also be grounded.
  • Each phase leg of the first converter 14 furthermore comprises a pair of converter valves.
  • the first phase leg PL 1 therefore comprises a first and a second converter valve CVA 1 and CVA 2
  • the second phase leg comprises a third and a fourth converter valve CVB 1 and CVB 2
  • the third phase leg PL 3 comprises a fifth and a sixth converter valve CVC 1 and CVC 2 .
  • the mid points of the phase legs are connected to corresponding AC terminals AC 1 , AC 2 , AC 3 , where each AC terminal is connected to a corresponding phase of the first AC system via the first transformer (not shown).
  • a phase leg is in this example divided into two halves, a first upper half and a second lower half, where such a half is also termed a phase arm. Thereby the phase legs are connected to the phases of the first AC system via the first transformer T 1 .
  • each valve pair is connect to a corresponding AC phase of the first AC system.
  • a phase arm between the first DC terminal DC 1 and a first, second or third AC terminal AC 1 , AC 2 and AC 3 may be termed a first phase arm or an upper phase arm, while a phase arm between the first, second or third AC terminal AC 1 , AC 2 and AC 3 and the second DC terminal DC 2 may be termed a second phase arm or a lower phase arm.
  • the phase arm midpoints are furthermore connected to the AC terminals via phase reactors LAC 1 , LAC 2 and LAC 3 .
  • the upper phase arms are joined to the first DC terminal DC 1 via a corresponding first or upper arm reactor LA 1 , LB 1 and LC 1
  • the lower phase arms are joined to the second DC terminal DC 2 via a second or lower arm reactor LA 2 , LB 2 and LC 2
  • the arm reactors are thus connected between a valve and a pole. It should however be realized that arm reactors may just as well be provided on the opposite side of the valves.
  • the first voltage source converter 14 may be a two-level converter, where each converter valve is made up of a number of series connected switching units.
  • the converter may be a modular multilevel converter (MMC) where each converter valve is formed through a series-connection or cascade of a number of submodules, where a submodule may be a half-bridge submodule or a full-bridge submodule.
  • MMC modular multilevel converter
  • a submodule then comprises one or two strings of series connected switching units in parallel with an energy storage element like a capacitor.
  • a switching unit may be realized in the form a transistor with anti-parallel diode.
  • IGCTs integrated gate-commutated thyristors
  • BIGTs Bi-mode Insulated Gate Transistors
  • a converter may for instance be an n-level converter, such as a neutral point clamped three-level converter.
  • a modular multilevel converter may be made up of a number of different types of submodules. There may also exits hybrid converters that use submodules in an n-level environment.
  • each AC terminal is thus connected in series with a corresponding current sensor that detects or senses the phase current is running through the AC terminal.
  • FIG. 2 only one such current sensor CS connected between the first AC terminal AC 1 and the first phase reactor LAC 1 of the first phase leg PL 1 is shown.
  • pairs of current sensors are instead used, where one current sensor in a pair is provided in the upper phase arm of a phase leg and the other current sensor in the pair is provided in the lower phase arm of the same phase leg.
  • control unit 26 which controls the operation of the first converter 14 and more particularly controls each converter valve.
  • the control unit 26 is provided for controlling all the phase arms of the converter. It should thus be realized that all converter valves are controlled by the control unit 26 .
  • FIG. 3 shows a block schematic of one way of realizing the control unit 26 .
  • the control unit 26 comprises a waveform control module WFC 28 and an active damping control module ADC 30 , where the waveform control module 28 is responsible for AC waveform generation and the active damping control module 30 is responsible for damping of oscillations between the first and the second converter. It can thereby be seen that the control unit controls the converter valves to generate at least one AC waveform and to reduce oscillations between the converters, which oscillations may be caused by resonances in the DC system. In the generation of the AC waveform the waveform control unit more particularly uses voltage and frequency control.
  • the second converter 16 may have the same realization as the first converter 14 .
  • the two converters in the same converter station can operate in frequency and voltage control mode (FVCtrl). This may as an example be the case when the HVDC link formed by the two poles 22 and 24 is used to black start a dead AC system or to transmit the power generated from remote/offshore windfarms to a main AC grid, where the dead AC system as well as the main AC grid would correspond to the first AC system ACB 1 .
  • FVCtrl frequency and voltage control mode
  • the two poles share the active power via a power frequency droop. They also share the AC voltage control via a voltage-reactive power droop.
  • the main advantage of operating both converters in FVCtrl is that the two poles can automatically share an active power imbalance (between load and generation) during grid restoration, or active power generated by wind farms, thanks to the power frequency droop functionality.
  • the second converter station 12 may be a supporting station controlling the DC voltage and may be connected to an AC system represented by an infinite source (the second AC system ACB 2 ), while the first converter station 10 is a black starting station in which both converters 14 and 16 operate in FVCtrl mode.
  • the two poles may oscillate against one another, where it is as an example possible that the frequency of oscillation is between 1 and a few hundred Hz.
  • the total power exchanged between the first AC system and the DC system may in this case be a more or less constant power without any oscillations. Therefore any oscillations observed in the output AC voltages of the first and second converters 14 and 16 would be oscillations occurring inside the HVDC system and between the two poles. The oscillations would thus be caused by resonances in the DC system.
  • the DC side of an HVDC system has the potential to form resonance at a number of frequencies due to submodule capacitance, pole capacitance and the phase arm inductance as well as inductance in the pole with extremely low damping.
  • aspects of the invention are therefore directed towards damping such oscillations between two converters.
  • the waveform control module 28 of the control unit 26 controls the converter valves so that an AC waveform is generated on each AC terminal AC 1 , AC 2 , AC 3 , where the waveform on an AC terminal may in a known fashion be separated by 120 degrees from the waveforms on the other AC terminals.
  • the waveform control module 28 is operating in FVCtrl mode and when operating in this mode it may also perform pulse width modulation control, such as Sinusoidal Pulse Width Modulation (SPWM) or 3 PWM.
  • SPWM Sinusoidal Pulse Width Modulation
  • the converter valves of a phase leg such as the first and second converter valves CVA 1 and CVA 2 of the first phase leg PL 1 , may receive a control signal representing a voltage level that it is desired to output.
  • a valve may thus receive a valve control signal Ur that reflects a desired voltage and frequency to be output on the first AC terminal AC 1 of the first converter 14 .
  • the active damping performed may be seen as the creation of a resistive voltage drop based on the phase current ia output from the converter, which voltage drop is provided in series with a voltage desired to be generated by the converter and may be resistive.
  • the active damping control module 30 may be seen as creating a virtual impedance, for instance a virtual resistance around any resonance frequencies formed between the two converters.
  • the damping is performed during the control of the valves. Therefore the description will be given in relation to the control of the first and the second valve CVA 1 and CVA 2 of the first phase leg PL 1 .
  • the same type of damping will take place also in respect of the second valve CVA 2 of the first phase leg PL 1 as well as in respect of the valves of the second and third phase legs PL 2 and PL 3 .
  • the active damping control module 30 comprises a d component control section and a q component control section.
  • FIG. 4 schematically shows a first variation of a d component control section 30 A of the active damping control module 30 and FIG. 5 shows a corresponding first realization of a q component control section 30 B of the active damping control module 30 .
  • the oscillations damping involves the creation of a virtual resistance, it involves the use of sensed phase currents in order to obtain the virtual voltage drop.
  • the operation is performed in the dq frame, it may be necessary to transform the sensed phase current ia into a d component phase current ia_d and a q component phase current ia_q.
  • the phase current ia may be transformed from an abc or static frame into a dq frame where d denotes direct and q denotes quadrature. There is thus a transforming from an abc frame to a direct and quadrature frame.
  • the transforming may involve creating an imaginary signal by shifting the original phase current signal ia by 90 degrees and this together with the original signal form alpha-beta waveforms, where the non-shifted waveform is the alpha waveform and the shifted waveform is the beta waveform.
  • the alpha waveform is multiplied with cos ⁇ , while the beta waveform is multiplied with sin ⁇ and added together in order to obtain the d component ia_d.
  • the alpha waveform is multiplied with sin ⁇ , while the beta waveform is multiplied with cos ⁇ in order to obtain the q component ia_q, were ⁇ is a suitable angle such as the phase angle.
  • the handling first of all involves processing the phase current so that a main power frequency component is excluded from a following operation, where the main power frequency component is the fundamental frequency component of the AC voltage output by the converter.
  • the processing involves high pass filtering the phase current so that the component is blocked.
  • the direct component of the phase current ia_d is provided to a first filter 34 that has a first cut off frequency.
  • the first filter is a high pass filter that high pass filters the d component using the first cut off frequency and provides it as a first d component processed current signal pc_d 1 .
  • the following operation of the handling from which the main power frequency component is excluded comprises multiplying the phase current with a proportional gain Kp. Therefore the first filter 34 provides the processed current signal pc_d to a first controller 36 .
  • the first controller 36 is a P controller performing proportional control with a gain Kp with respect to the processed current signal pc_d 1 in order to obtain an oscillations damping control signal dc_d representing the d component of the virtual voltage drop.
  • the first controller 36 thus applies proportional control on the filtered phase current in order to obtain the d component of the oscillations damping control signal dc_d.
  • the q component of the phase current ia_q is provided to a second filter 44 that has a corresponding first cut off frequency.
  • the second filter 44 high pass filters the q component using the first cut off frequency for obtaining a first q component processed current signal pc_q 1 that is provided to a second controller 46 .
  • the second controller 46 is also a P controller performing proportional control with a gain Kp with respect to the processed current signal pc_q 1 in order to obtain an oscillations damping control signal dc_q representing the q component of the virtual voltage drop.
  • the second controller 46 thus applies proportional control on the filtered phase current in order to obtain the oscillations damping control signal dc_q.
  • the damping control signals dc_d and dc_q may with advantage be combined with the waveform control of the phase legs and therefore the damping control signals may be in the form of voltages, which may then be combined with a control signal used by the waveform control module for controlling a phase arm.
  • the damping control signal may as an example be subtracted from the arm modulation indices, i.e. the different waveform control signals or converter reference voltages formed by the waveform control module 28 for use in the control of the converter valves of a phase leg for providing a waveform.
  • the oscillations damping control signal may thus be combined with the converter reference voltage in order to obtain the valve control signal.
  • the waveform control module 28 may thus already operate in the dq frame, which means that it may already have waveform references or converter reference voltages Uro_d and Uro_q in the dq frame, in which case the d component damping control signal dc_d may be simply subtracted from this d component reference voltage Uro_d and the q component damping control signal dc_q may be subtracted from the q component reference voltage Uro_q in order to obtain a valve control signals comprising the voltage drop. As the damping control signals are negative, this adding will result in a voltage drop.
  • the d component damping control signal dc_d is supplied to a first negative input terminal of a first subtracting element 38 , where the first subtracting element 38 has a second positive input terminal on which it receives a converter reference voltage Uro_d, often termed modulation index, used in the upper phase arm of the first phase leg.
  • the converter reference voltage Uro_d is generated by the waveforming control module 28 .
  • the first subtracting element 38 then subtracts the damping control signal from the converter reference voltage in order to obtain a d component valve control signal Ur_d for controlling the valves of the first phase leg.
  • the q component damping control signal dc_q is supplied to a first negative input terminal of a second subtracting element 48 , where the second subtracting element 48 has a second positive input terminal on which it receives a q component converter reference voltage Uro_q used in the upper phase arm of the second phase leg. Also this converter reference voltage Uro_q is generated by the waveforming control module 28 .
  • the second subtracting element 48 subtracts the damping control signal from the converter reference voltage in order to obtain a q component valve control signal Ur_q for controlling the valves of the first phase leg.
  • the two damping control signals Ur_d and Ur_q may then be used by the waveform control module 28 for supplying a valve control signal Ur in the abc or normal static frame to the valves CVA 1 and CV 2 .
  • the converter reference voltage is already provided in the normal static frame, i.e. that there is only one converter forming reference voltage Uro.
  • the damping control signal components may more particularly be converted back to the originally used static frame through inversely transforming the dq components.
  • the inverse transforming may thus involve transformation from the dq to the alfa-beta frame and from the alfa-beta frame to the static frame.
  • damping control signal may then be combined with a converter reference voltage in the static frame in order to obtain the valve control signal Ur.
  • the damping control signal may thus be combined with the arm modulation index, i.e. the converter reference voltage used by the waveform control module 28 .
  • valve currents in the upper and lower valves of a phase arm are measured instead of the phase current.
  • a phase current may then be formed as the difference between the two measured valve currents.
  • the phase current may thereafter be transformed into the dq frame and handled in the previously described manner.
  • the above described filters may have the same or different settings and may have cut-off frequencies in the range 1-70 Hz, so that the fundamental component of the static frame AC voltage, which in the dq frame corresponds to a DC component, is removed from the proportional control.
  • oscillations may generally occur in the range 1-a few hundred Hz. It can be seen that through the processing excluding a main power frequency component corresponding to the DC component in the dq frame together with a proportional control any oscillations in the arm current will be effectively damped. Thereby also power swings between the two converters will be dampened. Moreover, as the damping is performed using control it is possible to avoid introducing potentially expensive, bulky and lossy components such as resistors. The degree of damping may also be flexibly set using different values of the proportional gain. The control is also easily introduced in a converter through merely adding some software.
  • One variation is that more than one filtering of the phase current may be performed in the control.
  • FIG. 6 One example of this for the d component is shown in FIG. 6 .
  • FIG. 6 there is the first high pass filter 34 that receives and high pass filters the d component ia_d of the phase current and provides a first processed current signal pc_d 1 , just as in FIG. 4 .
  • the first processed current signal pc_d 1 is in this case provided to a first input terminal of an adding element 54 .
  • the second filter 50 receives the d component ia_d of the phase current and high pass filters it using the second cut-off frequency.
  • the filter 50 then provides the high pass filtered current as a second processed current signal pc_d 2 .
  • the second processed current signal pc_d 2 is therefore supplied to a second input terminal of the adding element 54 , which forms a sum of the two processed current signals and provides the sum to the first controller 36 . Thereafter proportional control and combination with reference voltage is made in the same was as was described above in relation to in FIG. 4 .
  • the filters may here be set differently.
  • the first filter may have a first cut-off frequency while the further filter may have a second cut-off frequency, where both may lie in the above described range of 1-70 Hz. It is here also possible that one cut off-frequency may lie closer to a first end of the range and the other may lie closer to the opposite end of the range.
  • This type of control may improve the damping even further.
  • the damping control signal was generated in the dq frame. It should be realized that this is not necessary.
  • the damping control signal may according to another variation be generated also in the static frame.
  • phase current ia is directly processed for obtaining the first processed current signal.
  • the processing may again involve high pass filtering.
  • the cut-off frequency is set differently than in the dq frame. It may as an example be set higher than the fundamental AC frequency. It may for instance be set higher than 50 or 60 Hz in order to remove the power frequency component.
  • the first and any other processed current signals are then supplied to a controller performing proportional and resonance control, which is thus a PR controller, where the resonance control may be performed at the fundamental frequency.
  • the output of the controller may then be combined with the static frame reference voltage for obtaining a valve control signal, which is then used for controlling the converter valves of a phase leg.
  • the second converter 16 has the same types of converter valves, current sensors and a control unit with the same type of control as the first converter.
  • control unit may be realized in the form of discrete components. However, it may also be implemented in the form of a processor with accompanying program memory comprising computer program code that performs the desired control functionality when being run on the processor.
  • a computer program product carrying this code can be provided as a data carrier such as one or more CD ROM discs or one or more memory sticks carrying the computer program code, which performs the above-described control functionality when being loaded into a control unit of a voltage source converter.
  • a data carrier such as one or more CD ROM discs or one or more memory sticks carrying the computer program code, which performs the above-described control functionality when being loaded into a control unit of a voltage source converter.
  • One such data carrier in the form of a CD Rom disk 56 carrying computer program code 58 is shown in FIG. 7 .

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CN109698514B (zh) * 2017-10-24 2022-07-22 南京南瑞继保电气有限公司 一种换流器控制方法及装置
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